Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a tracer gas which passes through the smallest of leaks in a sealed test piece. The helium is then drawn into a leak detection instrument and is measured. The quantity of helium corresponds to the leak rate. An important component of the instrument is a mass spectrometer tube which detects and measures the helium. The input gas is ionized and mass analyzed by the spectrometer tube in order to separate the helium component, which is then measured. In one approach, the interior of a test piece is coupled to the test port of the leak detector. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector.
One requirement of the spectrometer tube is that the inlet through which the helium and other gases are received be maintained at a relatively low pressure, typically below 2×10−4 Torr. Thus, leak detectors typically include a vacuum pumping system, which may include a roughing pump, a diffusion pump or turbomolecular pump and associated forepump, and a cold trap. Vacuum pumping systems for helium mass spectrometer leak detectors are described, for example, in U.S. Pat. No. 4,499,752, issued Feb. 19, 1985 to Fruzzetti et al. and U.S. Pat. No. 4,735,084, issued Apr. 5, 1988 to Fruzzetti.
A problem with helium mass spectrometer leak detectors is that the vacuum pumping system used to maintain the input of the spectrometer tube at the required pressure may have a low compression ratio for light gases, such as helium. As a result, helium in the ambient environment can move through the vacuum pumping system in reverse direction and be measured by the mass spectrometer. The helium that moves through the vacuum pumping system is not representative of a leak in the test piece and gives a false reading. This problem is exacerbated when helium is sprayed onto the test piece, thereby increasing the concentration of helium in the ambient environment and increasing the amount of helium that moves through the vacuum pumping system in reverse direction.
Scroll vacuum pumps have been used in helium mass spectrometer leak detectors. The scroll pump may be utilized as the roughing and/or backing pump. A scroll pump configured for backing a high vacuum pump in a mass spectrometer leak detector is disclosed in U.S. Pat. No. 5,542,828, issued Aug. 6, 1996 to Grenci et al.
Conventional scroll vacuum pumps have a relatively low compression ratio for light gases, such as helium. The compression ratio can be increased by reducing clearances and increasing the number of turns of the spiral scroll blades in the scroll vacuum pump. However, this approach substantially increases the cost of the scroll vacuum pump and is not acceptable for low-cost and/or portable applications.
Accordingly, there is a need for improved light gas vacuum pumping systems and methods.